If you have been signed up before you will not be able to signup again using a promotional code. Doing so would be a fraud, as DreamHost promotional codes are only available for use by "new, first time" customers.
If you have an account here previously, these code are not available to you and our signup system is pretty good about not even showing you a place to enter a code in such instances. (and this is by design).
Note that the promotional discount codes are designed to give new customers a chance to try our services at a price that is very low - even lower at times than what it costs to provide the service.
We do this because we think they will like the service and decide to stay at our still reasonable prices because they think the service is worth what we are asking.
You have experienced our service, so you now need to decide if you feel it is worth our standard price. I hope you will think that it is, and signup again. However, if you don't, we are not interested in providing you another deep discount to continue to use our services.
--DreamHost Tech Support